Alignment in the packaging of integrated circuits

ABSTRACT

A method includes aligning a top package to a bottom package using an alignment mark in the bottom package, and placing the top package over the bottom package, wherein the top package is aligned to the bottom package after the placing the top package over the bottom package. A reflow is then performed to bond the top package to the bottom package.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.13/922,130, entitled “Alignment in the Packaging of IntegratedCircuits,” filed on Jun. 19, 2013, which application is incorporatedherein by reference.

BACKGROUND

In a conventional Package-on-Package (PoP) process, a top package, inwhich a first device die is bonded, is bonded to a bottom packagethrough solder balls. The bottom package may also include a seconddevice die bonded therein. The second device die may be on the same sideof the bottom package as the solder balls.

Before the bonding of the top package to the bottom package, a moldingcompound is applied on the bottom package, with the molding compoundcovering the second device die and the solder balls. Since the solderballs are buried in the molding compound, a laser ablation or drillingis performed to form holes in the molding compound, so that the solderballs are exposed. The top package and the bottom package may then bebonded through the solder balls in the bottom package.

In the bonding process, the solder balls in the top package need to beaccurately aligned to the solder balls in the bottom package, so thatthe solder balls in the top package and the solder balls in the bottompackage may be reflowed to join to each other. To perform the alignment,alignment marks are formed in the reflow boat, wherein bottom packagesare placed in the openings in the reflow boat. The solder balls in thebottom packages are located through locating the alignment marks in thereflow boat. Alternatively, the solder balls in the top package and thesolder balls in the bottom package are used as alignment marks.

The conventional alignment schemes suffer from drawbacks. For example,the process variation in the sawing of the bottom packages may cause thevariation in the alignment of the solder balls. The surface of thesolder balls may be oxidized, making the identification of the solderballs difficult.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 10 are cross-sectional views and top views ofintermediate stages in the manufacturing of a Package-on-Package (PoP)in accordance with some exemplary embodiments; and

FIGS. 11A through 11I illustrate the top views of some exemplaryalignment marks.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A Package-on-Package (PoP) structure and the method of forming the sameare provided in accordance with various embodiments. The intermediatestages of forming the PoP structure are illustrated in accordance withsome embodiments. The variations of the embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements.

Referring to FIG. 1, bottom package 100 is formed, which includespackage component 10. In some embodiments, package component 10 ispackage substrate, and hence throughout the description, packagecomponent 10 is referred to as package substrate 10, although it can beof other types. In alternative embodiments, package component 10comprises an interposer. Package component 10 may be a part of a packagecomponent that includes a plurality of identical package components 10.For example, package component 10 may be a package substrate, and islocated in an un-sawed package substrate strip that comprises aplurality of package substrates formed as an array.

Package substrate 10 may comprise substrate 18 that is formed of adielectric material. Alternatively, substrate 18 may be formed of othermaterials such as a semiconductor material, for example, silicon. Insome exemplary embodiments, package substrate 10 is a build-up substratethat is built on a core, as shown in FIG. 1. Alternatively, packagesubstrate 10 is a laminate substrate which includes laminated dielectricfilms adhered together through lamination, wherein redistribution linesare embedded in the laminated dielectric films. When substrate 18 isformed of a dielectric material, the dielectric material may comprisecomposite materials that are mixed with glass fiber and/or resin.

Package substrate 10 is configured to electrically couple electricalconnectors 12 at first surface 10A to conductive features 16 at secondsurface 10B, wherein surfaces 10A and 10B are opposite surfaces ofpackage substrate 10. Electrical connectors 12 and conductive features16 may be metal pads, for example, and hence are referred to as metalpads 12 and 16, respectively. Package substrate 10 may includeconductive connections such as metal lines/vias 14, which may furthercomprise through-vias penetrating through substrate 18.

In some embodiments, electrical connectors 24 are formed at the topsurface of package component 10. Electrical Connectors 24 may beelectrically coupled to metal pads 12 and conductive features 16. Inexemplary embodiments, electrical connectors 24 are solder balls.Alternatively, electrical connectors 24 may comprise metal pads, metalpillars, solder caps formed on metal pillars, and/or the like. In someembodiments, the top ends of electrical connectors 24 are higher thantop surface 20A of die 20. Electrical connectors 24, when being solderballs, may comprise either eutectic solder or non-eutectic solder. Insome embodiments, the formation of electrical connectors 24 includes areflow process, wherein the solder-containing electrical connectors 24are reflowed to form solder balls.

Also referring to FIG. 1, package component 20 is bonded to packagesubstrate 10 through metal pads 12. Hence, bottom package 100, whichincludes package substrate 10 and package component 20, is formed.Package component 20 may be a device die, and is alternatively referredto as die 20 hereinafter, although it may also be another type ofpackage component such as a package. Die 20 may include integratedcircuit devices (not shown) such as transistors, capacitors, inductors,resistors, and/or the like. Furthermore, die 20 may be a logic circuitdie such as a Central Computing Unit (CPU) die. The bonding of die 20 tometal pads 12 may be achieved through solder bonding or directmetal-to-metal bonding (such as copper-to-copper bonding).

In some embodiments, alignment marks 26 are pre-formed on the backsideof die 20, which may be formed before the bonding of die 20 onto packagesubstrate 10, for example, during the manufacturing of die 20 and beforeit is sawed from the respective wafer. Alternatively, alignment marks 26are formed after the bonding of die 20 onto package substrate 10, andbefore the subsequently performed molding process. In the embodiments inwhich package component 20 is a device die, alignment marks 26 areformed on the backside of the respective semiconductor substrate (asilicon substrate, for example). Alignment marks 26 may be trenches thatextend from the back surface of package component 20 into anintermediate level of (the silicon substrate of) package component 20.In some embodiments, depth D1 of alignment marks 26 is between about 15μm and about 30 μm. It is appreciated, however, that the values recitedthroughout the description are merely examples, and may be changed todifferent values. Alignment marks 26 are used to mark the relativepositions of electrical connectors 24. Since the relative positions ofalignment marks 26 and electrical connectors 24 are fixed, by locatingalignment marks 26, the positions of electrical connectors 24 may bedetermined. In alternative embodiments, no alignment mark is formed onthe back surface of package component 20. Instead, alignment marks areformed in the subsequently formed molding material.

FIGS. 11A through 11I Illustrate the top views of a plurality ofcandidate alignment marks 26. Alignment marks 26 may have any top-viewshapes, providing they can be identified. For example, FIGS. 11A, 11B,11C, 11D, 11E and 11F illustrate a circle, a cross, a leftdouble-square, a right double-square, a diamond, and a square,respectively. FIGS. 10G, 10H, and 10I illustrate that alignment marks 26may have a grid shape (a double cross), a left diabolo shape, or a rightdiabolo shape, respectively.

Referring to FIG. 2, molding material 28 is molded on bottom package100. A curing process is performed after molding material 28 isdispensed. The top portions of electrical connectors 24 are over topsurface 28A of molding material 28. In some exemplary embodiments,height H1 of the upper portions of electrical connectors 24 that areover top surface 28A is greater than about ¼, or greater than about ⅓,of height H2 of electrical connectors 24. Ratio H1/H2 may also be closeto 0.5 in some embodiments. Molding material 28 may include a filler, apolymer, and a hardener in accordance with some embodiments. The polymermay be a molding compound, an underfill, a Molding Underfill (MUF), anepoxy, or the like.

In some embodiments, as shown in FIG. 2, molding material 28 has its topsurface 28A level with the top surface 20A of die 20, and hence die 20is exposed through molding material 28. The alignment marks 26 (if any)in die 20 are thus exposed and can be identified. In alternativeembodiments, molding material 28 may encircle and be in contact with thelower part of die 20, while the upper part of die 20 is over top surface28A of molding material 28. The alignment marks 26 (if any) in die 20are hence also exposed and can be identified. In alternativeembodiments, as shown in FIG. 4, molding material 28 may fullyencapsulate die 20 therein, with a portion of molding material 28 beingoverlapping die 20. In these embodiments, alignment marks are formed inmolding material 28.

In accordance with some embodiments, alignment marks 26 are formed afterthe molding of molding material 28. Alignment marks 26 may be formed inmolding material 28, wherein the respective alignment marks are shown as26A. The locations of alignment marks 26 are selected to avoid thelocations of electrical connectors 24, and can be in any position (inthe top view) of molding material 28. Alignment marks 26 may be formedthrough laser drilling in some embodiments. The top-view shapes ofalignment marks 26 may be selected from the exemplary shapes in FIGS.11A through 11I, or may be any other shapes that can be distinguished.Alignment marks 26 (marked as 26B) may also be formed on the backsurface of die 20 after the molding process.

FIG. 3 illustrate an exemplary top view of bottom package 100. Inaccordance with some embodiments, electrical connectors 24 are formed tosurround package component 20, although electrical connectors 24 may beformed with any other layout. In some embodiments, alignment marks 26include portions 26A formed in molding material 28, and portions 26Bformed in die 20. In alternative embodiments, alignment marks 26 includeportions 26A but not portions 26B. In yet alternative embodiments,alignment marks 26 include portions 26B but not portions 26A.

FIGS. 4 and 5 illustrate the formation of alignment marks 26 inaccordance with alternative embodiments. In these embodiments, as shownin FIG. 4, molding material 28 fully covers die 20 and electricalconnectors 24. Next, as shown in FIG. 5, the portions of moldingmaterial 28 that cover electrical connectors 24 are removed. The removalmay be performed using laser 30, which burns the portions of moldingmaterial 28 covering electrical connectors 24. In addition, laser 30 isalso used to form alignment marks 26, which extend from the top surfaceof molding material 28 into molding material 28. In these embodiments,alignment marks 26 may include portions 26A that are not directly overpackage component 20, and/or portions 26B that are directly over packagecomponent 20.

After the formation of bottom package 100, bottom package 100 is sawedfrom the respective package that includes a plurality of bottompackages. Bottom package 100 is placed into reflow boat 32, as shown inFIGS. 6 and 7, which illustrate a top view and a cross-sectional view,respectively. In some embodiments, reflow boat 32 includes plurality ofopenings 34 therein. Openings 34 are designed to accommodate bottompackage 100 therein, wherein the top view size and shape of bottompackage 100 is close to the top-view size and shape of the openings 34.FIG. 7 illustrates the cross-sectional view of reflow boat 32 and bottompackages 100, wherein the cross-sectional view in FIG. 7 is obtainedfrom the plane crossing line 7-7 in FIG. 6. In FIG. 7, bottom packages100 are schematically illustrated to show die 20 and some alignmentmarks 26. The remaining components of bottom packages 100 are notillustrated, and may be found referring to FIGS. 2, 3, and 5.

Next, referring to FIG. 8, top packages 200 are placed on bottompackages 100, which are located in reflow boat 32. FIG. 9 illustrates amagnified view of top package 200 and the respective bottom package 100.As shown in FIG. 9, top package 200 may be a package that includesdevice die 202 and package substrate 204, wherein device die 202 isbonded to package substrate 204. In some exemplary embodiments, devicedie 202 is a memory die such as a Static Random Access Memory (SRAM)die, a Dynamic Random Access Memory (DRAM) die, or the like.Furthermore, molding material 206 may be pre-molded on device die 202and package substrate 204 before top package 200 is placed on bottompackage 100.

In some embodiments, top package 200 includes solder balls 208 at thebottom surface of top package 200. Solder balls 208 need to be alignedto electrical connectors 24. The alignment is achieved through alignmentmarks 26, which is used to determine accurate locations of electricalconnectors 24. In some embodiments, the determination of the positionsof solder balls 208 is achieved by finding the positions of solder balls208. In alternative embodiments, alignment marks 226 are made on the topsurface of top package 200 for locating the positions of solder balls208. The formation of alignment marks 226 may be similar to theformation of alignment marks 26, and hence the details are not repeatedherein. In some embodiments, alignment marks 226 extend from the topsurface of molding material 206 into molding material 206. In theembodiments wherein device die 202 is exposed through molding material206, alignment marks may also be formed on the back surface (theillustrated top surface) of device die 202, and extends into thesubstrate of device die 202.

Next, the reflow boat 32 (FIGS. 7 and 8), bottom packages 100, and toppackages 200 go through a reflow process, so that bottom packages 100and the respective top packages 200 are bonded to form PoP structures.FIG. 10 illustrates a respective PoP structure. During the reflow,solder balls 208 and the solder in electrical connectors 24 (FIG. 9) aremolten, and merge with each other to form solder regions 36 as in FIG.10.

In the embodiments of the present disclosure, the alignment ofelectrical connectors such as solder regions are performed through thealignment marks made on the bottom package and/or the top package. Thepositions of the electrical connectors may thus be accuratelydetermined. As a comparison, in conventional alignment processes, thealignment is performed through the alignment marks on reflow boats. Theposition of the bottom packages may shift relative to the alignmentmarks on the reflow boats, for example, due to the process variation inthe cutting of bottom packages. The shift in the positions of the bottompackages, however, is not taken into account when the alignment isperformed. The alignment accuracy in conventional alignment process ishence adversely affected.

In accordance with some embodiments, a method includes aligning a toppackage to a bottom package using an alignment mark in the bottompackage, and placing the top package over the bottom package, whereinthe top package is aligned to the bottom package after the placing thetop package over the bottom package. A reflow is then performed to bondthe top package to the bottom package.

In accordance with other embodiments, a method includes bonding a devicedie onto a package substrate to form a bottom package, dispensing amolding material to mold at least a lower portion of the device die inthe molding material, and forming an alignment mark in at least one ofthe molding material and the device die.

In accordance with yet other embodiments, a package includes a devicedie over and bonded onto a package substrate, and a molding materialmolding at least a lower portion of the device die therein. The moldingmaterial is overlying the package substrate. A solder region is at a topsurface of, and electrically coupled to, the package substrate. Analignment mark is at a top surface of one of the device die and thepackage substrate.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A package comprising: a bottom packagecomprising: a package substrate; a device die over and bonded to thepackage substrate; a molding material encapsulating at least a lowerportion of the device die therein, wherein the molding material extendsoverlying the package substrate, wherein a top surface of the device dieis exposed through the molding material; a solder region at a topsurface of, and electrically coupled to, the package substrate, whereinthe solder region comprises a lower portion in the molding material, andan upper portion protruding out of, and higher than, a planar topsurface the molding material; and a first alignment mark at a topsurface of the molding material, wherein an entirety of the firstalignment mark is lower than the upper portion of the solder region; anda top package bonded to the solder region, wherein first sidewalls ofthe lower portion of the solder region are smooth and rounded, andsecond sidewalls of the upper portion of the solder region are smoothand rounded, and wherein the first sidewalls are joined to the secondsidewalls with a discontinuity in smoothness, and the discontinuity isat a level substantially coplanar with the planar top surface of themolding material, wherein the top package further comprises: a secondalignment mark extending from a top surface of the top package to anintermediate level of the top package; an additional device die; and anadditional molding material encapsulating the additional device die,wherein the second alignment mark is formed in the additional moldingmaterial.
 2. The package of claim 1, wherein the upper portion has arounded top surface.
 3. The package of claim 1, wherein the solderregion is an edge solder region that is closest to an edge of the bottompackage than all solder regions on the package substrate and at a samelevel as the edge solder region, and the first alignment mark is betweenthe edge solder region and the edge of the bottom package.
 4. Thepackage of claim 1 further comprising a second alignment mark, whereinthe second alignment mark further comprises an additional trenchextending from the top surface of the device die into the device die. 5.The package of claim 4, wherein the additional trench is located at acorner of the device die.
 6. The package of claim 1, wherein the firstalignment mark comprises a trench extending from the top surface of themolding material into the molding material, and a top end of the firstalignment mark is coplanar with the top surface of the device die.
 7. Apackage comprising: a package substrate; a device die over and bonded tothe package substrate; a solder region physically joined to the packagesubstrate, the solder region comprising: a lower portion having firstsidewalls, wherein the first sidewalls are smooth and rounded; an upperportion comprising second sidewalls, wherein the second sidewalls aresmooth and rounded, and wherein the first sidewalls and the secondsidewalls are joined to each other with a discontinuity in smoothness; amolding material molding the device die and the lower portion of thesolder region therein, wherein the device die, the package substrate,and the molding material form a first package, wherein the upper portionof the solder region protrudes out of a planar top surface of themolding material, with a joint of the lower portion and the upperportion of the solder region being coplanar with the planar top surfaceof the molding material; a first alignment mark extending into one ofthe device die and the molding material; and a second package bonded tothe first package through the solder region, wherein the second packagecomprises: an additional device die; an additional molding materialencapsulating the additional device die; and a second alignment mark,wherein the second alignment mark is formed in the additional moldingmaterial.
 8. The package of claim 7, wherein a top end of the firstalignment mark is coplanar with the joint of the lower portion and theupper portion of the solder region.
 9. The package of claim 7, whereinthe first alignment mark comprises a trench extending into the devicedie.
 10. The package of claim 9, wherein the trench of the firstalignment mark is located at a corner of the device die.
 11. The packageof claim 7, wherein the planar top surface of the molding material iscoplanar with a top surface of the device die.
 12. The package of claim7, wherein the first alignment mark is in the molding material, and thefirst alignment mark overlaps the device die.
 13. The package of claim7, wherein the first alignment mark is in the device die, and a portionof the first alignment mark is at a same level as a portion of themolding material.
 14. A package comprising: a package substrate; adevice die over and bonded to the package substrate; a solder regioncomprising: bottom surface attached to the package substrate; and acontinuously rounded surface comprising rounded top surfaces and roundedsidewalls; an encapsulating material encapsulating the device dietherein, wherein the encapsulating material comprises a top portionoverlapping the device die; and an alignment mark comprising a trenchextending into the top portion of the encapsulating material.
 15. Thepackage of claim 14, wherein an entirety of the alignment mark is higherthan the solder region.
 16. The package of claim 14, wherein the roundedtop surfaces of the solder region have a spherical shape.
 17. Thepackage of claim 14, wherein no package component is over and bonded tothe solder region.
 18. The package of claim 14 further comprising anadditional alignment mark not overlapping the device die.
 19. Thepackage of claim 14, wherein the encapsulating material comprises anopening, and an entirety of the solder region is in the opening.
 20. Thepackage of claim 14, wherein the alignment mark is located at a cornerof the package.